The expanding demand for optical coherence tomography (OCT) in medical and industrial markets has driven unprecedented advances in high-speed, widely-tunable, narrow-linewidth, semiconductor lasers. These new devices present a unique enabling technology for industrial distance metrology applications.
Interferometry is a useful tool for distance measurement since the only fundamental limit to measurement precision is photon statistics. The LIGO interferometer is a good example of the precision that can be reached with interferometry with care (and a lot of money), providing sensitivity to displacement noise densities of less than 10−6 pm/Hz1/2. In practical industrial applications however, measurement precision can often be limited due to other effects such as environmental or electronic noise or the measurement/processing techniques employed.
Three measurement techniques typically used in OCT include time-domain OCT (TD-OCT), and spectral domain techniques, including Fourier domain OCT and Swept Source OCT (SS-OCT). In TD-OCT, an interference signal from a spectrally broad source is generated by displacing a reference mirror in the interferometer. In FD-OCT, interference of various spectral components of a spectrally broad source is observed with a spectrometer, In SS-OCT, the interference is sampled rapidly during a high speed sweep over a broad optical spectrum using a tunable laser. The high speed sweep can include a sequence of measurements at different, discrete frequencies or a continuous sweep through a continuum of frequencies. In the case of a stepped laser having different, discrete frequencies, each “step” can correspond to a new lasing mode that results in a different optical frequency.
Spectral domain techniques have been proven to have greater sensitivity than time domain techniques due to the fact that they are capable of collecting signals from all sample depths during the entire acquisition time. This advantage coupled with a method for determining the absolute wavelength and a physical-model based analysis provides significantly improved distance measurements for industrial distance measurement applications.